Lundell alternators are used almost exclusively for passenger vehicle on-board electrical generation. A Lundell alternator includes a wound-field claw pole rotor that has the characteristic of satisfying most functional demands such as power output, size, and reliability in a cost effective manner. In a dynamoelectric machine having such a rotor, it is necessary to apply a voltage across the rotating field coil winding to produce a time varying magnetic field. A slip ring and brush arrangement is used to complete an electrical circuit between the rotating field coil winding and the stationary electronics of the alternator, particularly the voltage regulator.
In such an arrangement, two copper slip rings are mounted on a rotating shaft extending from the rotor and are electrically insulated from the shaft and each other through plastic material that supports the rings. Each ring is connected to one end of the field coil winding. The rings are cylindrical in shape and roughly about 5 to about 10 mm in axial length. An electrical brush rides, or more appropriately, slides, on a respective rotating slip ring. The two brushes are secured to a stationary frame of the alternator via a brush holder or similar structure, and are forced into direct, sliding contact with an exposed surface of a corresponding slip ring. A biasing member, such as a spring, is used to bias each brush in contact with a corresponding slip ring.
The two brushes are commonly solid carbon, or carbon-copper pieces that are typically rectangular in shape. The brushes are electrically connected to the rest of the alternator to provide the voltage across the field coil. During use, the slip rings rotate with the rotor shaft and the two brushes slide across the exposed surface of a corresponding slip ring to complete the electrical field circuit between the field coil winding and the alternator.
The electrical interface between the brush and respective slip ring, however, is not perfect. The brushes do not always make continuous, low-impedance contact with the slip rings due to vibration, surface roughness and roundness of the slip rings coupled with the rotational speeds of the rotor. Furthermore, the inductance of the field coil winding can lead to negative voltage spikes within the field circuit. Consequently, localized arcing and sparking occur between the slip rings and corresponding brushes. This in turn produces electrical noise that can be disruptive to other on-board vehicle electronics, such as the radio.
Accordingly, it is desired to suppress the electrical noise generated due to the aforementioned voltage spikes that occur as a result of the imperfect slip ring/brush interface.